Diamond is of strong interest as a potential semiconductor material for high voltage, high frequency, and/or high power active and passive electronic devices because of its superlative materials properties, including high electronic carrier mobilities, high breakdown field strength, high thermal diffusivity, favorable matrix for quantum devices, as well as many other desirable optical, chemical, and materials properties. However, a major barrier to exploiting diamond for active electronic applications is that there are no dopants known that have a sufficiently low thermal activation energy barrier to create a concentration of electronic carriers and a carrier mobility in diamond at room temperature that is adequate for most devices of interest.
While there are many known defect and impurity states in the wide bandgap (5.45 eV) of diamond, several of which can act as donors or acceptors of electronic charge, only boron (creating an acceptor state) and phosphorous (creating a donor state) have been demonstrated to be reliable dopants. Boron, the most commonly used diamond dopant, has the smallest activation energy of 0.37 eV at low doping concentrations (<1017 cm−3). However, this activation energy is still high enough to ensure that only a fraction of the boron present is activated at room temperature, leading to relatively low concentrations of free carriers.
Increasing the boron concentration in diamond reduces the activation energy, such that at a concentration of approximately 3×1020 cm−3 the metal-to-insulator transition point occurs and a fully-activated impurity band is formed via the quantum tunneling of holes between neighboring boron acceptor states. Unfortunately, as the activation energy of the holes decreases, so does carrier mobility, not only because of the increased impurity scattering but also due to the onset of a low-mobility, hopping-like conduction. The resulting material is one that has sub-unity carrier mobility and typical sheet carrier densities in excess of those that are readily controlled, for example, by a typical field effect transistor (FET).
One approach to creating both high mobility and high carrier concentrations for electronic materials in two dimensions is the formation of “nanometric delta doped” layers, which are heavily doped layers, typically less than five nanometers in thickness, that are located adjacent to high mobility intrinsic material, so that a fraction of the carriers created by the heavily ionized dopant layer reside in the adjacent high mobility layer.
The success of “delta doping” requires the epitaxial growth of a very thin, heavily doped “delta layer” that is typically between 1 and 2 nm thick and is preferably doped to a concentration that is above the metal insulator transition, which for boron in diamond means a concentration of at least approximately 4×1020 cm−3. Successful delta doping further requires that the interface between the doped “delta” layer and the high mobility intrinsic layer (containing less than 1017 cm−3 boron atoms) must be abrupt, and must also be atomically smooth, so as to minimize carrier scattering.
Recent attempts at delta doping of diamond with boron have failed to demonstrate the theoretically expected performance, and have shown low carrier mobilities, low sheet carrier concentrations, and/or low channel mobility. Some of these studies have attributed this disappointing performance to poor lateral homogeneity and interrupted morphology of the delta layers. Carrier mobilities measured in these studies did not exceed the range of 1 to 4.4 cm2/Vsec which is typical of bulk diamond doped with boron above the metal insulator transition level of 4 to 5×1020 cm−3. These results are well short of the values of approximately 100 cm2/Vsec mobility and 1013 cm−2 sheet carrier concentrations that are required for the implementation of doped layers of diamond in practical electronic devices.
What is needed, therefore, is an apparatus and method for creating nanometric delta doped layers in epitaxial diamond with interfaces between the doped and high mobility layers that are sufficiently abrupt and smooth to provide at least 100 cm2/Vsec carrier mobility and 1013 cm−2 sheet carrier concentrations.